On Sun, 21 Dec 2008 15:38:40 -0500, hpeer wrote:

I have Volvo MD7A, 13 hp two cylinder unit with salt water cooling.

Runs perfectly with one exception, it occasionally overheats and I can't
figure out what the heck is going on. I bought the boat used and the
previous owner had fitter her out himself. Every time she overheats, I
find something to "fix" after which she is fine for a spell then she
will overheat again. Clearly I'm not getting to the root cause.
Generally she will seem to be running along just fine and all of a
sudden the pressure gauge just goes up, up, up. It seems to be a
bi-state kind of thing, it either works perfectly well or not at all.
Its almost as if there is a ball valve or something somewhere that is
sticking. It's obviously not a bad gage.

The plumbing goes like this:
Through hull fitting
Salt water strainer
Tap to the transmission/engine
Tap to secondary pick up from bilge
Tap to sinks


Here is some of the history and what I had done, in each case I have
also checked/changed the impeller. They usually look just perfect.

1: Recently fitted salt water strainer seemed to be blocked with debris
from its construction. It is home built out of some interlocking pieces
of PVC pipe, the inner piece has a zillion small holes drilled in it. I
dumped this stuff out and she was fine.

2: Found a small leak in the copper pipe from the transmission to the
impeller. I thought that maybe it was sucking in air and the impeller
was getting air bound. Repaired the leak.

3: Moved and lowered the salt water strainer. This was mounted pretty
high on a bulkhead with real long hoses. I moved it to the engine
compartment and lowered it. I thought that maybe there was just too
much suction head to get the water going.

4: Threw thermostat overboard.

Any clues?

Many thanks,

Howard


I am the process of rebuilding my own raw water cooling and have the
following comments about yours:

The usual configuration for the engine cooling is Thru-hull to
Strainer to xmission cooler to water pump to oil cooler to engine to
exhaust elbow and overboard.

You used the word "tap" to indicate connections to the transmission
cooler. This should be the entire flow from the strainer to the pump,
i.e., hose from strainer to xmission cooler to pump. The hose from the
thru-hull to the strainer to the cooler to the pump and onward to the
engine should be the full size of the thru-hull (if possible).

If you are going to install "taps" in the line for uses other then
engine cooling you MUST be sure that these taps are isolated with
valves and not used while the engine is running.

The main strainer should/must be located below the water line so that
with the sea cock open the strainer fills.

If this is an old engine and it has not been flushed after every use
in salt water, which is seldom/never done you probably have scale
built up inside the engine that could be blocking water passages. This
scale will not dissolve in fresh water, by the way. There are acid
flushes which will remove it but I do not have a formula. Perhaps
google if you feel this might be a problem.

Regarding water coming out the exhaust. It is not an accurate
indicator of anything except that water is getting through. In my own
case there ~appears~ to be normal amounts of water out the exhaust but
the engine overheated over 1800 RPM due to restrictions in the inlet
line to the pump.

By the way, you DO want a strainer in the line to keep stuff out of
the system. I have removed plastic bags from the main strainer as well
as small creatures, weeds, mud, small sticks and innumerable things
that you probably don't want in the engine or pump.
Cheers,

Bruce
(bruceinbangkokatgmaildotcom)